Stuck helical on Petri 1.9 rangefinder

[luisalegria]

Stuck helical - what to do ?

I have a nice Petri rangefinder with a nice big lens, yet another "poor mans Leica". I can get he whole thing working except for the focusing - the helical is stuck as hard as I have seen it. I have disassembled the whole thing completely, so now I just have the thing on a brass mounting plate with little else to damage. The thing seems to have been lubricated with the dreaded green stuff that turns into epoxy glue. Its all brass, plate, outer sleeve, inner sleeve, helical threads, etc.

I have tried -
- WD-40
- Soaking in lighter fluid for two days
- Soaking in vinegar
- Soaking in alcohol
- Using a cable wrench and then a real pipe wrench, holding the mounting plate in a vise.
- Using a solvent for stuck screws
- Ultrasonic water bath with detergent.

The only things left seem to be heat ( a blowtorch ? oven ?) or acids ?

Any ideas ?

[fuzzywuzzy]

There's commercial penetrating oil.

I googled DIY penetrating oil and found several car enthusiast forums recommending a homebrew recipe for penetrating oil - half acetone, half automatic transmission fluid.

I tried it on some wee screws that WD40 wouldn't budge, had good luck with it. I was cautioned that acetone might be hard on finishes but you're past caring by now I bet

Some of the car guys said that automatic transmission fluid all by itself works well, it's so thin that it creeps into every nook and cranny.

Acetone evaporates quickly so I think this would be something to mix as you needed it.

Good luck, let us know how it works out!

EDIT - I'd be scared to put a torch to brass, the melting point is low and it wouldn't take much to soften and deform things

[luisalegria]

Thanks for that. I have tried several oils; lighter fluid (naptha) does indeed easily get into everything, its my favorite substance really, and almost always works.

But I think what I need to do is find a solvent for this &#^&*@%$(@ old grease.

Maybe acetone ?

[David]

I would boil it first. Set something on the bottom of the pot to keep the brass part from directly contact the heat. This could be a few large nuts (the metal kind) or do a double boil. If that doesn't work, pressure cooking might work. Of course, for both options the lens would need to be entirely dismantled.

How old is the lens? I'll see if any of the chemical or mechanical engineers at work have any ideas. Knowing the age would allow us to guess at the grease's composition. For instance, a non-hydrocarbon grease may be inert to many or all of the compounds you used.

[luisalegria]

Boiling does sound like a likely next try, thanks.

Now to find a pot to use that my wife won't kill me for boiling oily mechanical parts in.

[NikonD]

or maybe freezing it and immediately sinking it into boiling water
acetone would probably help or maybe if you can get Dichloromethane (methylene chloride)... it's extremely aggressive on paint, grease, silicone...

copy from wikipedia:
Dichloromethane's volatility and ability to dissolve a wide range of organic compounds makes it a useful solvent for many chemical processes. Concerns about its health effects have led to a search for alternatives in many of these applications. It is widely used as a paint stripper and a degreaser

[David]

So I checked with an environmental engineer and grease from before WWII would be solely hydrocarbon-based. Greases with fluorocarbons and other nasty, thinner-resistant chemicals weren't developed until the 1950s. Interestingly, the entire modern chemical industry owes its origins to research performed by the Nazis. But I digress. So this lens is from before WWII, I assume, so the grease is definitely a hydrocarbon-based compound that may be little more sophisticated than axle grease. He agreed that boiling, so long as the brass didn't rest on the pot bottom, would probably be the best solution. In short, if the lighter fluid didn't thin the grease, then it wasn't able to reach the grease.

[David]

Ever get the helical unstuck?

[luisalegria]

Yep, I did get it unstuck.
Heated with a butane torch.

Unfortunately I have since found out that the other underlying problem with Petri rangefinders is that the cocking linkage on the lens side is brass, and gets worn out easily. Poor materials and careless design. A pity as I think the shutter and lens are perfectly fine, as well as the fit and finish.

I have three of these Petri's now, of two different models, and all have the same problem. I am thinking of replacing/reinforcing the part with a steel piece, but thats for another day.

I strongly discourage purchasing these Petri rangefinders, they are very likely to disappoint.